1. Field of the Invention
This invention relates to an apparatus with constant liquid spray rate, and more particularly to a dispensing monitor to maintain a constant liquid spray rate by controlling the pressure of the gas used to drive the liquid.
2. Description of Related Art
A silicon substrate used in semiconductor fabrication usually has an uneven surface, the result of the number of dielectric layers formed over the substrate to cover some structure such as a contact window or a capacitor. Semiconductor fabrication therefore needs a planarization process to obtain an even substrate surface. Otherwise, becau0 se of the differences in relative elevation, the interconnecting metal lines can not easily be formed and the precision of transferring a desired pattern onto the substrate is compromised.
Currently, a technology of spin-on glass (SOG) is widely used for a local planarization on the substrate surface. SOG uses a solution that includes a dielectric material in a solvent for spin coating on the substrate. The dielectric material in the solution fills the concave structures on the substrate to obtain a local planarization.
The spray rate of the SOG solution is usually quite so low that a special apparatus is necessary to precisely control the spray rate. FIG. 1 is a schematic setup of a conventional Tokyo Okika Kggyo CO.LTD (TOK) SOG machine. In FIG. 1, there is a container 2, which contains a SOG solution and a gas. A special cover 4 covers the container 2. Helium (He) gas is pumped into the container 2 through a duct 40 and the cover 4 in order to pressurize container 2, which propels the SOG solution out through the cover 4 and a duct 42. There is a valve 6 on the duct 42 to control whether the SOG solution is exported or not. When the valve 6 is open, SOG solution is driven to a nozzle 8, located at the terminal end of the duct 42. The nozzle 8 sprays SOG solution onto a semiconductor substrate 44 for planarization on the substrate surface. Moreover, in general, in order to measure and control the internal pressure, a pressure sensing apparatus 11 close to a pressure switch 10 is mounted on the duct 40 to indirectly measure the internal pressure of container 2. In this case, if there is any damage to the duct 42, the damage cannot be detected with the result that the spray rate cannot be precisely controlled. This is a serious drawback of the TOK SOG machine.
FIG. 2 is a schematic setup of a conventional Dai Nipon Screen CO.LTD (DNS) SOG machine. The like reference numbers represent the like objects. The DNS SOG machine is similar to the TOK SOG machine except for the container 2. In FIG. 2, a container 22 contains a SOG solution and is located inside a pressure barrel 12. The container 22 is compressible and can be compressed by the pressure barrel 12 to propel the SOG solution out through the cover 4 and the duct 42. The pressure barrel 12 is pressurized when N.sub.2 is supplied along the duct 40. Since the pressure sensing apparatus 11 is in the same location as in the TOK SOG machine, both SOG machines have the same problem as described above in TOK machine.